'Huge blow'

The head of Lebanon's Internal Security Forces, Major-General Ashraf Rifi, described Mr Hassan's death as a "huge blow" and warned that further attacks were likely.

"We've lost a central security pillar," he told local media.

"Without a doubt, we have more sacrifices coming in the future. We know that, but we will not be broken."

More than 30,000 people have been killed in Syria since a Sunni-led popular uprising against Mr Assad, a member of the Shiite-linked Alawite sect, broke out 19 months ago.

International powers fear the conflict could inflame rivalries across the region as it intensifies.

Lebanon's religious communities are divided between those supporting Mr Assad and those backing the rebels and the country had already felt the heat prior to Friday's bombing.

Sunnis and Alawites have clashed in Tripoli while the northern end of the Bekaa Valley, which borders Syria, has suffered from shelling and incursions.

Iranian foreign minister Ali Akbar Salehi, whose country is a powerful backer of Mr Assad and Hezbollah, condemned the bombing and said he planned to visit Beirut on Saturday.

The Iranian Foreign Ministry, in a statement on its website, suggested Israel was to blame for the attack.

A senior Israeli official dismissed the suggestion as "beyond pathetic".

'Dangerous sign'

Lebanese troops reinforced road junctions and official buildings in Beirut on Saturday and the government met to mull a response to the car bomb killing.

Sunni Muslims took to the streets and burned tyres across Lebanon over Friday night and Saturday morning to protest against the bombing, which revived memories of the carnage of Lebanon's own civil war.

Two people were wounded when the Lebanese army opened fire on a group that was trying to block a road to protest against the bomb attack.

The Red Cross put the toll from the attack at four dead and number hurt at 110.

"This is not the final toll, as we still need to see what happens to the critically wounded," Red Cross spokesman Ayad Mounzer said.

Protesters blocked roads to the international airport and in the northern, mostly-Sunni city of Tripoli.

Rallies were also held and roads closed in the eastern Bekaa Valley and in the southern town of Sidon.

At a roadblock in south Beirut, a dozen unidentified gunmen in civilian clothes were seen standing next to the burning tyres.

Soldiers and police guarded street corners in Beirut's Ashrafiyeh, the mainly Christian district where the bomb exploded during rush hour, and at Martyrs' Square in the centre.

The Beirut Star newspaper said the perpetrators clearly aimed to force Lebanon into a new round of chaotic violence.

"If the goal was to divert attention from the events in Syria, then people should remember this well and head off any attempt to take Lebanon further into tension and civil strife," it said.

United States secretary of state Hillary Clinton said Mr Hassan's killing was "a dangerous sign that there are those who continue to seek to undermine Lebanon's stability".

French president Francois Hollande urged Lebanese politicians to stay united and prevent attempts to destabilise the country.